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“How Many Ways to Measure Moisture?”

Posted by Art Gatenby on Dec 11, 2008 12:38:00 PM

People often ask me, “How Many Ways are there to Measure Moisture?”

I’ll try to give you the answer and a quick description of the four main ways.

  1. The earliest and still most widely used method is weight loss on drying. In this technique, the sample weight at the start is recorded. After drying the sample weight is recorded. The difference in these weights represent the moisture lost. People use many combinations of equipment to get at this widely performed test. some people use a balance and a hot plate, others a balance and an oven. Today technology has made it easy to use an integrated balance and drying method to automate the procedure.
  2. Another system for moisture detection is based on the fact that many materials change electrical or dielectric characteristics depending the moisture level in the material. Most of these instruments measure changes in resistance , conductivity or capacitance. Because these techniques are measuring an indirect affect of moisture, calibration is necessary. The calibration is accomplished by comparing the dielectric reading to a known moisture for the sample. A graph is prepared and used to provide the translation of the electrical characteristic to the amount of moisture in the material sample. In automated instruments these tables are processed by a computer to give reading in % moisture..
  3. A third method is based on a chemical reaction and is known a Karl Fischer Titration. This is named after the chemist who developed the chemical/reagent. The reagent reacts with water to eliminate the conductivity that was developed by the water in the test sample. This test is accomplished by injecting the material into a solvent and then adding the Karl Fisher Reagent. The reagent is added until the reaction has converted all the water into a non conductive chemical. The amount of reagent used is measured and converted into units of water. Karl Fischer techniques are advantageous for measuring small amounts of water, for samples that contain volatiles that degrade the accuracy of weight loss on drying techniques and for releasing bound water in some materials.
  4. Application of the concepts of reflection and absorption of electromagnetic energy is becoming widely used, particularly for in-line or on-line measurement. These techniques based on the finding that water (as well as other chemicals) has very specific absorption wavelengths. Near Infrared (NIR) and Microwave technology are the most commonly used. Although the techniques of NIR and microwave are different they are both based on the concept of absorption of electromagnetic energy. In these tests the absorption of the wave lengths associated with water are compared to a reference wave length, When the moisture of a calibration sample is known the absorption results can be referenced to moisture. The use of lasers for this purpose is a newer related technique.

With this link you can look at examples of equipment used in each of these categories.

There are several other techniques that involve creating a chemical reaction with water to develop pressure (speedy Moisture Tester is a prime example) or super heating the sample and measuring the volume/weight to the water that was driven off.

This got a little longer than I planned. As you can see moisture detection and measurement is a wide ranging application.


Topics: Moisture

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